Brand and model on the current hard wired nonnetworked ones?
BTW, if what you are looking for is primarily away from home notification, kidde has a really interesting solution which is a plug in networked device that just listens for the other alarms to go off and that is capable of sending notification. It’s intended for a retrofit situation. Gets decent reviews.
It’s not a smoke alarm itself, it will not trigger the other smoke alarms, but it gives you a notification capability for your existing alarms, which is often the main update people are looking for.
In the US, by code, smoke alarms have a very specific acoustic pattern, which is why this works. I just don’t know if it’s the same pattern in UK. It runs on Wi-Fi.
Because it can send multiple text notifications, you can use IFTTT for indirect integration with smartThings now in the U.S., and once that is fixed for the UK.
I like this one for a lot of retrofit situations, and if you’ve read my other posts you know that fire safety is a big deal for me. This is just a nice clean approach. There is one thing to watch out for, which is like a lot of Wi-Fi devices, it really works a lot better when it is not connected through a wifi extender, but that’s something you can test with him and see if placement makes a difference.
So that’s just one option. Literally plug and play.
http://www.homedepot.com/p/Kidde-RemoteLync-Home-Monitoring-Device-21026465/205843011
As for your relay idea, sure, several people have done that. Usually with a relay and a door sensor. The issue with that is you have to match it based on model. And you’re paying for two devices anyway. So the plug-in ends up costing about the same, and should work any sensor that meets the US code.
You can search the forums, there are multiple topics on using the relays. Here’s a good one. I’d add any additional questions about the relay method to that topic.